1. Field of the Invention
This invention relates to a drum brake device and more specifically to a dual mode type drum brake which functions as a leading-trailing (LT) type brake device during the service brake operation and functions as a duo servo (DS) type brake device during the parking brake operation.
2. Description of Related Art
Related art dual mode type drum brake devices are, for example, disclosed in U.S. Pat. No. 5,275,260, Japanese patent publication number 62-8652 (AU-B1-53 491/79 for English) and Japanese provisional patent publication number 9-273573 (U.S. Pat. No. 6,003,645 for English).
General structures of the Related art dual mode type drum brake devices are disclosed in the aforementioned publications and therefore the explanation of which is set forth in an example to be explained hereunder with reference to FIGS. 6 and 7.
A back plate 1 has a central hole 1a freely fitting over an axle of a vehicle, and four bolts, not shown in the figures, are inserted through four bolt holes 1b for a purpose of fixing the same on a stationary portion of the vehicle.
A pair of facing first and second brake shoes 2 and 3 is provided, each makes a T-shape in cross-section as a shoe rim 2a is formed together with a shoe web 2b while a shoe rim 3a is formed together with a shoe web 3b, and linings 2c and 3c are adhered on peripheral surfaces of the shoe rims 2a and 3a respectively.
The brake shoes 2 and 3 are movably mounted on the back plate 1 by conventional shoe-hold mechanisms 4 and 5, each composed of a plate spring and a pin.
In FIG. 6, a wheel cylinder 6 functioning as a service brake actuator is arranged between adjacent upper ends 2d and 3d of the brake shoes 2 and 3, which is fixed on the back plate 1.
An anchor block 7 is arranged between adjacent lower ends 2e and 3e of the brake shoes 2 and 3, which is fixed on a protrusion of the back plate 1 by two rivets 8 and 8.
An extensible shoe clearance adjustment device 9 composed of an adjustment bolt 10, an adjustment nut 11 and a socket 12, extends between the pair of brake shoes 2 and 3.
Tools such as a screwdriver may be inserted from an insertion hole 1c (FIG. 7) formed on the back plate 1 or a hole formed on a brake drum, not shown in the figures; and adjustment teeth 10a formed on the adjustment bolt 10 are manually rotated to screw the adjustment bolt 10 in or out from the adjustment nut 11 so as to adjust the shoe clearance.
Although FIGS. 6 and 7 disclose an automatic shoe clearance adjustment mechanism composed of an adjustment lever 13, an adjustment spring 14 and a pin 15, the explanation of which is limited to an anti-vibratory function to the pivot lever 16 and the parking brake actuator 17.
The pivot lever 16 is constantly urged to be clockwise, via the pin 15 which also pivotally supports the adjustment lever 13 and is integrated with the pivot lever 16, due to a spring force of the adjustment spring 14, thereby suppressing the vibration of the pivot lever 16 and the parking brake actuator 17.
A central region of the pivot lever 16 or a portion of the pivot lever 16 between an abutment point with the shoe clearance adjustment device 9 and an abutment point with a later described strut 20 in FIG. 6 is pivotally supported on the first brake shoe 2 around a pivot section 16c. An upper end 16a and a lower end 16b of the pivot lever 16 respectively make a functional engagement with the shoe clearance adjustment device 9 and the strut 20.
A parking brake actuator 17 composed of a brake lever 18, the strut 20 and a pin 21 is provided adjacent to the anchor block 7.
The brake lever 18 is designed such that the proximal portion 18a of the brake lever 18 is pivotally supported on the second brake shoe 3 around the pin 21 adjacent to the anchor block 7. A cable end 23a on the tip of the brake cable 23 arranged through a guide pipe 26 fixed on the back plate 1 is hooked and rested on a free end 18b of the brake lever 18 at the wheel cylinder 6 side.
The strut 20 extends between a portion on the brake lever 18 adjacent to the proximal portion 18a and the lower end 16b of the pivot lever 16.
Further, a shoe return spring 24 is stretched between the upper ends of the brake shoes 2 and 3 while a shoe return spring 25 is stretched between the lower ends of the brake shoes 2 and 3, urging the brake shoes 2 and 3 to approach each other, and upper facing portions of the brake shoes 2 and 3 engage and abut against the shoe clearance adjustment device 9 while lower facing ends 2e and 3e engage and abut against the anchor block 7.
Brake operation of the above-described structure will be explained next.
First, a service brake operation will be explained.
If a brake pedal, not shown in the figures, is stepped on to pressurize the wheel cylinder 6, the adjacent upper ends 2d and 3d of the brake shoes 2 and 3 are pressed to spread apart from each other pivoting with abutment points between the adjacent lower ends 2e and 3e and the anchor block 7, and the linings 2c and 3c frictionally engage with the brake drum, not shown in the figures, thereby generating a rotating braking force and functioning as a leading-trailing type drum brake.
Next, a parking brake operation will be explained.
A hand lever, not shown in the figures, is operated to pull the brake cable 23 toward the first brake shoe 2 side, and the brake lever 18 rotates clockwise in FIG. 6 around the pin 21 to press the strut 20. The pressing force on the strut 20 rotates the pivot lever 16 counterclockwise around the pivot section 16c spreading the second brake shoe 3 open via the shoe clearance adjustment device 9. The pivot lever 16 rotates counterclockwise with an abutment point between the pivot lever 16 and the shoe clearance adjustment device 9, thereby spreading the first brake shoe 2 open due to the pressing force acting on the pivot section 16c of the pivot lever 16.
A reaction force of the brake lever 18 acts on the lower end 3e of the second brake shoe 3 via the pin 21 with abutment point between the brake lever 18 and the strut 20.
Accordingly, linings 2c and 3c of both brake shoes 2 and 3 frictionally engage with the brake drum, not shown in the figures, thereby keeping the stationary state of the brake drum.
A frictional force of a primary brake shoe 2 or 3, being caused by a rotational direction of the brake drum, is transmitted to the remaining secondary brake shoe 3 or 2 via the shoe clearance adjustment device 9, thereby functioning as a duo servo type drum brake.
The above-described related art dual mode type drum brakes have the following drawbacks:
FIG. 8 is an example of a part of chassis at a rear side of the vehicle.
FIG. 8 illustrates drum brakes a and a attached on two-end side surfaces of an axle housing b. In FIG. 8, the left drum brake a only describes a back plate 1 and a wheel cylinder 6 and a right drum brake, explained in FIGS. 6 and 7, only describes a back plate 1. Each wheel cylinder 6, only one of which is illustrated in the left drum brake in FIG. 8, is generally positioned at an upper central portion of the back plates 1, 1 in order to avoid damaging a brake pipe and brake hoses c, c in case of a fallen wheel in a gutter, flying gravel, or the like.
In the above-described related art dual mode type drum brake, the brake cable 23 is arranged adjacent to the wheel cylinder 6 and extended toward a front side of the vehicle through the guide pipe 26.
Shock absorbers d and d and coil springs e and e existing at a front side of the vehicle become obstacles for a routing design of the brake cable 23.
This invention was made to improve the above-points and is to provide a dual mode type drum brake with an improvement in a simple structure of a parking brake actuator, in which a brake cable can be arranged in a lower side of a drum brake relative to an axle, thereby not being interrupted by any chassis parts.
In order to achieve the above-described objectives, this invention provides a dual mode drum brake wherein a parking brake actuator is designed such that a first brake lever has a proximal portion pivotally supported on a portion between both ends of a second brake shoe and a second brake lever has a proximal portion pivotally supported on a lower end portion of the second brake shoe. A free end of the first brake lever and a free end of the second brake lever extend so as to be substantially parallel and facing each other. The free end of the second brake lever functionally engages with the first brake lever between the proximal portion and the free end of the first brake lever. A strut extends between the second brake lever at a portion between the two ends thereof and the lower end side of the pivot lever; and a brake cable to be pulled to activate the free end of the first brake lever is arranged adjacent to the anchor and extended toward the first brake shoe side.
The dual mode drum brake of this invention may be designed such that the second brake lever is composed of one mono-plate, the proximal portion thereof is formed into two parallel facing forked legs to put the shoe web of the second brake shoe therebetween and a section thereof from an intermediate portion adjacent to an inner circumference of the shoe web to the free end thereof makes an almost C-shape in cross-section.
Then, an external surface of a bottom portion or an internal surface of a bottom portion of the C-shaped section functionally engages with the first brake lever.
According to the above-structured dual mode type drum brake device, a position on the back plate, from which the brake cable pulling the first brake lever is penetrated, is located at an anchor side. In other words, a position on the back plate, from which the brake cable is arranged to pass through, is located at a lower side of the axle. Therefore, no chassis parts become obstacles when routing the brake cable.
Both the first and the second brake levers may be simply integrally-formed by pressing, and the proximal portion of the second brake lever formed into forked legs putting a shoe web therebetween increases the strength against a biasing force generated due to an operation force of the first brake lever. At the same time, the operation force is transmitted to the strut without being biased.
Further, a lever-ratio of the brake lever is a value obtained by multiplying lever-ratios of the first brake lever and the second brake lever, which allows a low lever-ratio of each brake lever and enables a design of the brake levers to be thinner and lighter.